Taek-Soo Kim

Polymer thin films in wearable electronics undergo creep recovery during operation as on-body devices, resulting in unrecoverable strain that subsequently induces mechanical instability in the films. However, the creep recovery behavior of polymer thin films has not been investigated due to their intrinsic fragility and the difficulty in controlling ultralow loads for the films. We evaluate the creep recovery behavior of pseudo-free-standing polymer thin films as a function of creep stress, developing direct creep recovery testing with a feedback-based ultralow-load control system. Polystyrene (PS) thin films exhibit a sharp increase in unrecoverable strain with the onset of crazing. While poly(3-butylthiophene) (P3BT) thin films show a linear increase in the relative ratio of unrecoverable strain with increasing creep stress, poly(3-hexylthiophene) (P3HT) thin films exhibit a viscoelastic strain larger than unrecoverable strain. This work reveals the creep recovery behavior of polymer thin films and proposes a direct creep recovery testing method.

The interfacial fracture behavior of particulate-filled epoxy composites bonded to inorganic surfaces determines how well these bonded structures resist delamination. However, it has generally been analyzed by treating the composite as a homogeneous material, without considering the role of the fillers near the interface. Here, the interfacial fracture of a silica-filled epoxy molding compound (EMC) bonded to four die surfaces in high-bandwidth memory (HBM) packages—Si, SiO₂, SiNₓ, and Cu—was investigated to clarify how the substrate influences the participation of fillers in interfacial fracture. Asymmetric double cantilever beam tests gave critical interfacial fracture energies, Gc, in the order Si (25.8 J/m²) ≈ SiO₂ (24.8 J/m²) > SiNₓ (17.8 J/m²) > Cu (12.6 J/m²), and the cross-sectional EMC residue area fraction on the substrate side followed the same order (9.6%, 9.4%, 3.8%, and 2.7%). Although a continuum-level finite element model predicted a nominal crack-kinking tendency toward Si, the crack remained near the interface and was deflected around silica fillers retained on the substrate. Electron microscopy and X-ray photoelectron spectroscopy revealed an oxygen-containing filler–Si interface and substrate-dependent chemical environments (Si–O–Si, Si–O–N, and Si–O–Cu). Composite-level simulations showed that increased filler participation along the crack path promoted crack deflection and increased the loading work by 42%. These results indicate that substrate-dependent filler anchoring governs the retention and mechanical participation of fillers near the interface and thereby the interfacial fracture resistance of silica-filled epoxy composites.

A modulus-gradient shear-lag design which hardens substrate and softens perovskite toward an intermediate modulus, reduces stress and cracks in stretchable perovskite solar cells, yielding 22.14% efficiency with 80% retention at 60% strain.

The molecular weight (MW) of conjugated polymers (CPs) plays a fundamental role in the formation of polymer chain networks, which determines the mechanical, electrical, and chemical properties of the polymers. For stretchable electronics, superior mechanical elongation and strength of CPs in a thin-film state can be obtained when polymer chains exceed their critical MW (Mc), resulting in polymer chain entanglement and tie molecules. However, the organic stretchable devices are inevitably subjected to thermal stress as temperature changes in actual operation and processing. Despite the high thermal expansivity of stretchable polymer substrates, few studies have been conducted on thermomechanical reliability of the stretchable devices. Herein, we investigated thermomechanical reliability issues of stretchable organic semiconductors with CP thin films. Stretchable devices consisting of the CP films with different MWs on the poly(dimethylsiloxane) (PDMS) substrate were subjected to a thermal stress test. After the test, thermal cracking occurred in the CP film having MW below Mc due to its brittleness, resulting in a dramatic electrical mobility drop (more than 1 order of magnitude). In contrast, the high-MW CP films (MW > Mc) formed randomly oriented wrinkles after the test, and the decrease in the electrical mobility of the stretchable device was lower than that of the films with lower MW. To analyze the thermomechanical stabilities of stretchable devices, both the tensile tests of CP thin films with different MW and finite-element simulation of the stretchable device deformation as the temperature changes were performed. It was confirmed that although the high-MW CP films are subjected to higher thermal stress than that of the films with lower MW, the high-MW films can maintain electrical pathways without mechanical fracture by the formation of wrinkles due to the plastic deformation. Our study demonstrates that employing CPs with high MW above its Mc enables the fabrication of thermomechanically robust stretchable devices.

In advanced heterogeneous packaging, interposers serve as key components, integrating multiple chiplets onto a single package substrate through high-density signal redistribution. However, incorporating high-CTE metallic interconnects within low-CTE dielectrics and substrates inevitably induces significant warpage during thermal cycling. This deformation poses a severe threat to mechanical reliability, potentially leading to interconnect failure and yield loss, particularly as package dimensions increase while bump pitches and silicon die thicknesses scale down. While accurate warpage prediction is crucial for robust design, conventional finite element analysis (FEA) faces prohibitive computational challenges due to the extreme geometric scale mismatch between fine routing features and the global package structure. To address this challenge, this study proposes a deep learning–accelerated multiscale framework for rapid and accurate warpage prediction of interposer routing interconnects. The proposed approach establishes a warpage prediction pipeline by representing the spatial heterogeneity of complex routing patterns through equivalent properties. Validation against full routing-level simulations confirmed that the proposed method precisely predicts warpage behavior under optimal partitioning conditions. Furthermore, a deep learning surrogate model was employed to replace the iterative FEA-based homogenization process. The trained model enables rapid property estimation while maintaining high physical consistency with routing geometries. Consequently, the integrated methodology achieves a 2,643-fold speedup in analysis time while preserving the accuracy of full routing-level simulations. Ultimately, this framework provides a scalable and resource-efficient simulation strategy that minimizes computational overhead for the precision design of advanced packaging.

Stretchable displays suffer image distortion due to transverse compression of substrates under uniaxial stretching. Although auxetic frameworks with negative Poisson's ratios, which expand vertically during horizontal stretching, offer promising solutions, their void-rich structures are unsuitable for displays, requiring hybrid integration with continuous elastomeric substrates. This integration is challenging because rotational motions essential to auxetic unit cells can induce local substrate deformation, while the substrate can also restrict rotation, undermining auxetic performance. Here we show that design methodologies for hybrid auxetic metamaterial platforms, in which auxetic frameworks and elastomeric substrates are bonded at selective sites optimized for both horizontal and vertical stretching, enable isotropic expansion across local-to-global scales. To this end, the isotropic expansion factor is introduced as a statistical metric for quantifying image distortion. With the proposed hybrid platforms, we demonstrate stretchable displays exhibiting uniform, distortion-free expansion and stable performance under repeated stretching.